This invention relates to Ethernet architecture in a 100 megabits/second (Mbps) environment. More particularly, in a specific embodiment, the invention relates to Class II Ethernet stackable repeaters.
Class II Ethernet repeaters are the class that has a sufficiently low latency so that they can be connected together to form a 205 meter diameter, 100 Mbps Ethernet network according to the IEEE 802.3 U standard (currently in Draft 4). In 100 Mbps Ethernet network, a Class I repeater allows only one repeater between any two stations in the network; whereas, a Class II repeater due to its low latency allows two repeaters between any two stations in the network.
The "latency" of a repeater is the total transmission delay of signals through the repeater. Low latency is important in 100 Mbps Ethernet repeaters, because the bit budget is very tight. In a 10 Mbps Ethernet network, four repeaters are allowed between any two stations in the network. The number of repeaters permitted between any two stations in the network is derived from the collision slot time and the delays through the network. A slot time is the maximum time it takes to detect a collision, which equals twice the propagation time for the maximum cable length. The slot time in a worst-case collision scenario on a valid 10 Mbps Ethernet network of maximum size is 51.2 microseconds. In the 10 Mbps Ethernet environment, a slot time of 51.2 microseconds is 512 bit times. However, in the 100 Mbps Ethernet environment, 512 bit times corresponds to a slot time of 5.12 microseconds, i.e, one tenth the slot time in the 10 Mbps Ethernet environment, barring a change in the Ethernet algorithm or packet size. Thus, the bit budget is thus very tight, making low latency very important in 100 Mbps Ethernet repeaters.
One typical Class II Ethernet repeater is non-stackable and has 16 ports, allowing up to 30 stations in a 100 Mbps Ethernet network. After using one port on each repeater to connect together two of these Class II Ethernet 16-port repeaters, the remaining 15 ports on each repeater are available for connection to stations. In 100 Mbps Ethernet, it is desirable to increase the number of stations able to be connected to the network (up to the permitted maximum of 1,024 stations). There is room for improvement in terms of providing a repeater with reduced latency to constitute a Class II repeater and that allows for a greater number of stations in a 100 Mbps Ethernet network.
Other conventional Ethernet stackable repeaters, but which are not Class II repeaters, use a daisy-chained architecture with TTL. The term "stackable" means that multiple copies of identical stackable repeater modules may be connected together and physically stacked to form one logical stacked repeater. Even one such stackable repeater module may itself be used as a logical repeater. This type of stacked repeater architecture is achieved by use of wired-OR interconnection of 4-bit data, data envelope, data clock, ANYXN, and binary ACTN signal lines in the backplane. For this type of stacked repeater daisy-chained architecture, the latency includes the daisy chaining of the acknowledge signals through the repeater module. As the number of repeater modules in the stack are increased, the latency of the logical stacked repeater increases proportionately. Thus, conventional Ethernet stackable repeater modules have been limited in height to five per stack as a Class I repeater, due to the latencies involved in the daisy-chained architecture. It is desirable to have stackable repeaters with a greater amount of repeater modules per stack to permit more stations to connect to the network.
Another conventional Ethernet repeater providing multi-connect capability in a 10 Mbps Ethernet network is a non-stackable repeater having a single chassis with multiple media cards plugged into it. That is, different media cards plug into the same backplane, which includes a serial data line implemented as a differential open collector pair. In this repeater which uses Transistor-Transistor Logic (TTL), the single serial data line is Manchester-encoded, making a clock and envelope signal unnecessary. The backplane of this single chassis repeater has a link activity detect (LAD) signal used to detect the number of ports that are active in the repeater. The LAD signal line is implemented with terminations of 330 ohms and 220 ohms to about +5 V at each end of the backplane in the single chassis repeater. Each card has the ability to pull down the LAD line with its respective open collector NAND gate (such as a 74F38) through a series resistance of about 60 ohms coupled to the LAD signal line. This series resistance matches the characteristic impedance of the line. When zero ports are active, the LAD signal line is at its "quiescent" state of about +3 V. When one port is active, the LAD signal line is at its "one port active" state of about +1.5 V. When greater than one port is active, the LAD signal line is at its "greater than one port active" state of about +1.0 V. However, this kind of non-stackable repeater may not have sufficiently low latency to be a Class II repeater in a 100 Mbps Ethernet environment. It also has the problem of limiting the size of the network, given the relatively small number of ports corresponding to the single chassis, non-stackable repeater. It is desirable to provide a stackable repeater usable as a Class II or Class I repeater that allows flexibility as well as expandability in size in the environment of a 100 Mbps Ethernet network to account for growth of the network in an economically feasible manner.